1. Field of the Invention
The invention relates to a photographic paper support with a water resistant coating of polyethylene.
2. Brief Description of the Background of the Invention Including Prior Art
Water resistant photographic paper supports comprise a photographic paper base with synthetic resin layers applied to both sides. The synthetic resins can comprise polyolefins such as polyethylene and can be applied onto the paper by an extrusion coating method. Alternatively, the synthetic resins can comprise an organic varnish or lacquer mixture, which is applied to the paper by a dipping or spraying process. These coatings are dried, solidified and stiffened with heat or a high energy radiation. After resin coating and further treating of the surface light sensitive coatings, which can be one or several layers and are normally based on a silver halide, are applied to the front side synthetic resin layer. The light sensitive layers can be either black and white photographic layers or color photographic layers.
The front side synthetic resin layer disposed under the light sensitive layers usually comprises light reflecting white pigments as well as possibly color pigments, optical brightening agents and/or additives such as anti-static agents, dispersing agents for the white pigment, anti-oxidizing agents, release agents, external lubricants and the like. In the following, the term auxiliary agents includes color pigments, optical brightening agents, anti-static agents, dispersing agents for a white pigment, anti-oxidizing agents, release agents and external lubricants.
The synthetic resin layer applied on the side of the paper base opposite to that used for receiving the light sensitive layers (back side coating) can contain or be free of pigments and/or it can contain other additives, which would correspond to the use and employment of the laminate in each case as a photographic support and which can in principle correspond to those additive agents employed in the front side coating.
The front side coating can have additional functional layers, which, for example, improve relevant properties such as the adhesion of a light sensitive layer.
The back side coating can further comprise additional functional layers, which, for example, improve the possibilities of writing on the surface, the anti-static properties, the sliding properties or the planarity of the laminate.
German Patent Application Laid Out No. 16 19 233 teaches the use of polyethylene mixtures for providing layers of tape shaped material with extruder coating. This reference teaches general background aspects of interest in connection with the present invention.
The coating of a photographic paper base with polyolefin by way of meltextrusion coating through a wide slot die is known in the art. For example, U.S. Pat. No. 3,411,908 teaches such a system as well as other aspects generally employed in the preparation of resin coated photographic paper. It is also known that certain difficulties occur in an extrusion system of polyethylene, which can lead to substantial interferences, disturbances or even to unsuitability of the resulting photographic paper support material in view of the high sensitivity of photographic processes.
Disadvantages occur in particular in the case of polyethylene of low density. Such disadvantages include in particular an increased tendency for adhesive attachment of the layers when the paper is wound up onto a roll. Further, in the case of glossy surfaces, specks and so-called "secondary pits", are generated due to the high pressure inside the paper roll and upon separation of the layers during unwinding. Secondary pits are indentations originating from the contact of a rough back surface with a glossy front side. A further disadvantage is observed in an insufficient stiffness of the product.
Disadvantages occur even when employing polyethylenes of higher densities (HDPE). In particular, substantially decreased adhesion strength of the paper base, caused by the higher melt viscosity and the higher crystallinity of the resin in the coating, is observed. These disadvantages can be corrected within limits by increased extruder temperatures and/or lower operating speeds of the production machines.
Further disadvantages of the high density polyethylene (HDPE) as compared with the low density polyethylene (LDPE) include increased inhomogenities, so called gel-particles, which are small but clearly visible agglomerates of cross-linked or coagulated high molecular parts of the polyethylene as well as the requirement for additives needed to counteract the catalyst residues in the HD-polyethylene.
In addition, high density polyethylene cannot be extended and stretched to layer thicknesses of from about 10 to 50 micrometers at the desired higher operating speeds. Otherwise, defects, imperfections and holes occur in the molten film. Secondary disturbances due to increased extruder temperatures include, for example, dark colored particles that are generated by degraded, decomposed, disintegrated or burnt polyethylene. Furthermore, agglomerates, the called gel-particles, pinholes, pits or specks result in disturbances during application of light sensitive silver halide emulsions. These appear in the developed photograph as areas of decreased, changed or completely lacking black or, respectively, color density. Catalyst residues in the polyethylene can further influence the sensitivity of photographic layers, and this can become visible only after an extended storage time. In order to limit the disadvantages that are particularly caused because of the use of the high density polyethylene (HDPE), one can employ mixtures of low density polyethylene (LDPE) and high density polyethylene (HDPE).
The density range of the polyethylene types employed is for low density polyethylene (LDPE), between from about 0.914 and 0.926 gram per cubic centimeter, and for high density polyethylene (HDPE), between from about 0.950 to 0.965 gram per cubic centimeter. Mixing these types allows the setting of nearly any density value desired (DAS 1619233).
Despite the mixing of the two polyethylene types, the problems, which are in particular associated with the high density polyethylene (HDPE), cannot be fully eliminated. In this category fall the gel-particles and other agglomerates formed, as well as the catalyst residues present in the high density polyethylene (HDPE) and their photochemical effects. A further disadvantage of both the low density polyethylene (LDPE) and of the high density polyethylene (HDPE) types, as well as mixtures therefrom, is that a possible content in whitening agent or white pigment, usually titanium dioxide TiO.sub.2, is limited to about 12 weight percent unless substantially more difficult processing conditions are accepted.